Various charging apparatuses capable of ozone-less charging or ultimate ozone reduction and remarkable low-voltage charging without modifying a principal part of a photosensitive member and capable of sufficient contribution from the viewpoints of environment, electrical safety and electric power cost have been proposed.
For example, Japanese Laid-Open Patent Application (JP-A) 2003-084545 discloses an image forming apparatus in which a charging apparatus includes a light source for emitting light for generating an electric charge in the charge generating layer, a light transparent electrode (light transmissive electrode) through which the light from the light source is passable, and a bias voltage applying means for applying the bias voltage to the light transparent electrode. Then, the bias voltage is applied to the light transparent electrode while irradiating a certain area of a photosensitive drum with light from the light source via the light transparent electrode, so that a charged state is kept at a downstream side of a light irradiation area with respect to a movement direction of the photosensitive drum. Then, on the surface of the photosensitive drum in the charged state, an electrostatic latent image is formed by an exposure unit.
JP-A Hei 04-186380 discloses a charging apparatus, used in an image forming apparatus for forming an image by electrostatic latent image technology, for charging a photosensitive member which has an internal polarization effect. The charging apparatus includes a charging belt which has a medium-level volume resistivity and is an endless flexible light-transmissive (transparent) member. The charging apparatus further includes first to third rollers which are provided and spaced apart from each other, which act so as to bring the charging belt into intimate contact with the photosensitive member and which have volume resistivity levels lower than that of the charging belt. The first roller is grounded. The second roller is biased so that it has the substantially same potential as a polarization surface potential of the charging belt but has an opposite polarity to that of the polarization surface potential. The third roller is biased so that it has the substantially same potential as the polarization surface potential of the charging belt and has an identical polarity to that of the polarization surface potential. The charging apparatus further includes a light source, provided between the first and second rollers, for irradiating the photosensitive member with light through the charging belt. Further, the charging belt and the photosensitive member are moved, at their intimate contact portion, in the same direction at the same speed.
In the above-constituted charging apparatus for the photosensitive member, between the first and second rollers, the surface of the photosensitive member is charged to a potential of an opposite polarity to the charge polarity of the second roller, i.e., to the polarization surface potential by a potential, having a gradient, generated in the charging belt and by the light irradiation from the light source.
Further, between the second and third rollers, the potential of the charging belt varies from the polarization surface potential of the opposite polarity to the polarization surface potential of the identical polarity. However, the photosensitive member is not subjected to the light irradiation and therefore the charged state of the surface is kept as it is. When the charging belt is separated (spaced) from the photosensitive member after passing through the third roller, the charging belt and the photosensitive member have the same potential and therefore electric discharge is not caused, so that a harmful substance is not generated.
Further, when the first to third rollers are each constituted by an electroconductive roller having an elastic surface, it is possible to provide a tension to the charging belt so as to be intimately contacted to the photosensitive member.
However, as in the above-described conventional constitutions, in the case of an internal polarization charging type in which an electron pair is generated on the photosensitive drum by reception of the light energy, an intimate contact property between the electrode member and the photosensitive drum is important. In the case where the intimate contact property is insufficient, there arose a problem that image defect such a white dropout (patch) or non-uniformity was caused by electric discharge at the intimate contact portion. Further, in the constitution of JP-A 2003-084545, the light transparent electrode is not intimately contacted to the surface of the photosensitive drum. Further, in the constitution of JP-A Hei 04-186380, the charging apparatus itself is complicated and upsized, thus failing to lead to downsizing and cost reduction required for the image forming apparatus in recent years.